What alexandrite actually is
Alexandrite is the chromium-bearing color-change variety of the mineral species chrysoberyl[IMA][GIA]. The chemistry is a beryllium aluminum oxide, formula BeAl₂O₄, with chromium substituting at the aluminum site at parts-per-thousand levels. Chromium is the chromophore that drives the color-change effect[IGS]. The species crystallizes in the orthorhombic system. Habit is short prismatic with characteristic cyclic twinning that produces pseudo-hexagonal aggregates. Trillings (three-fold twins) are the most diagnostic crystal form for natural alexandrite[USGS][Gem-A].
The species relationship to other chrysoberyl varieties is chromatic: alexandrite is the chromium-bearing color-change member, cymophane is the chatoyant-only yellow-green chrysoberyl with no color-change, ordinary chrysoberyl is the yellow to greenish-yellow common variety[IGS][CIBJO]. The boundary between alexandrite and ordinary chrysoberyl is defined by the color-change effect itself, without a measurable shift between daylight and incandescent light, the gem cannot be called alexandrite under modern trade nomenclature[LMHC].
The optical fingerprints labs use to confirm species are the refractive index range 1.745–1.755, birefringence 0.008–0.011, specific gravity 3.71–3.75, dispersion 0.015, conchoidal fracture, vitreous to subadamantine luster, and distinct cleavage in one direction[IGS][USGS]. Optic sign is biaxial positive[USGS].
Color science
The color-change effect in alexandrite is a chromium-mediated optical phenomenon. Chromium substitution in the chrysoberyl lattice produces an absorption band centered near 580 nanometers that splits the visible spectrum into two roughly equal transmission windows: green (around 500–525 nm) and red (around 640–680 nm)[GIA][IGS]. The perceived hue depends on the spectral content of the light source: daylight, rich in shorter-wavelength blue and green, emphasizes the green transmission window. Incandescent light, weighted toward longer-wavelength red, emphasizes the red transmission window. The same gem appears bluish green under daylight and red to purplish-red under incandescent.
Pleochroism in alexandrite is exceptionally strong. The same crystal shows three distinct colors along the three optical axes: green, orange, and purplish red[IGS][GIA]. Lapidaries orient the rough so that the strongest color-change is visible face-up. This requires careful examination of the rough under both light sources before cutting commits to an axis orientation. The combination of strong pleochroism and color-change makes alexandrite one of the most cut-sensitive species in the trade.
The grading scale for color-change rates the percentage of color shift between daylight and incandescent appearance. Russian-type material from the original Ural deposits achieves close to a hundred-percent shift from pure green to pure red. Brazilian Hematita and Bahia material approaches this. Sri Lankan material typically shifts forty to sixty percent, often with olive-green daylight and brownish-red incandescent appearance[GRS].
Origin science
Geographic origin determination for alexandrite is a specialized lab practice grounded in inclusion microscopy and trace-element chemistry[SSEF][Gübelin]. Reports issued today by SSEF, GRS, GIA, and Gübelin combine both channels. Origin opinions can differ when diagnostic features overlap.
Russian Ural alexandrite, the original 1830s discovery, shows the strongest color-change and a distinctive inclusion suite of healed channels and chromium-rich growth zones. Production from these deposits has been commercially exhausted since the late nineteenth century, but Russian provenance carries the highest auction premium when a stone passes lab certification[IGS].
Brazilian production from Hematita (Minas Gerais) and Bahia is the modern commercial leader. The strongest Brazilian material rivals Russian quality in color-change percentage and saturation, though the deposits are increasingly worked out[IGS]. Sri Lankan alexandrite typically shows weaker color-change with olive-green daylight and brownish-red incandescent appearance. The production is steady but the material rarely reaches collector grade[IGS]. Tanzanian Tunduru and Madagascan production rounds out the modern supply, with variable color-change strength depending on the specific deposit.
Geological context separates the source families: Russian material formed in mica-schist-hosted hydrothermal contact zones, Brazilian production is from granitic pegmatite-related schist deposits, Sri Lankan material is alluvial, rolled from primary deposits long since eroded[USGS].
Treatment science
Alexandrite is overwhelmingly untreated in commerce[GIA][IGS][Lotus]. Heat treatment, the dominant enhancement for sapphire and ruby, does not improve alexandrite's color-change effect. The chromium-mediated absorption band that drives the optical phenomenon is set during initial crystallization and cannot be modified post-formation by atmosphere-controlled furnacing. High-temperature exposure can in fact damage the color-change effect by altering the chromium oxidation state.
Oil and resin filling of fractures is documented but uncommon in fine alexandrite commerce[Lotus]. The clean, fracture-free habit of premium alexandrite combined with the species' overall scarcity means that low-clarity faceted material rarely commands the price premium that would justify treatment investment.
Synthetic alexandrite is a documented market presence. Czochralski-pulled synthetic alexandrite from Russian and Japanese growers, flux-grown synthetic alexandrite from Inamori (Kyocera) and other producers, and hydrothermal synthetic alexandrite from Russian growers all reach the trade[Gem-A]. Detection at a major lab relies on inclusion microscopy (curved striae from melt-grown material, flux residues, platinum from crucible contact) and spectroscopy that identifies subtle differences in trace-element ratios versus natural deposits[Lotus][SSEF].
Phenomena science
Color-change, the defining phenomenon of alexandrite, is reflection-and-transmission switching driven by chromium substitution in the chrysoberyl lattice[GIA][IGS]. Under standardized D65 daylight illumination, the gem transmits primarily in the green portion of the visible spectrum. Under standardized A-illuminant incandescent light, the gem transmits primarily in the red. The mechanism is the chromium absorption band centered near 580 nm splitting the visible spectrum into two roughly equal transmission windows. The dominant transmission window then matches the dominant emission peak of the illuminating source. Color-change is rated on a percentage scale from weak (twenty to forty percent shift) through moderate (forty to seventy percent) to strong (seventy to one-hundred percent)[GRS].
Cat's-eye alexandrite, a chatoyancy-plus-color-change combination, is reflection from oriented parallel rutile-silk inclusions in the prismatic plane combined with the color-change effect[GIA][IGS]. The phenomenon requires cabochon faceting. Flat-domed orientation preserves both the color-change axis alignment and the silk-reflection geometry. Cat's-eye alexandrite combines two of the rarest gemological phenomena in a single stone and commands a substantial premium over either phenomenon alone.
Species identity synthesized from the authoritative sources cited below.